According to the "simple systems" approach, the mechanisms underlying a set of complex phenomena can be most easily elucidated by concentrated research on the simplest system which manifests those phenomena. It is argued in this proposal that two modulatory phenomena studied by cognitive psychologists--the speeding of reactions by alerting and orienting--are manifested in a simple system that has been at least moderately well characterized at the neurophysiological level, the startle-blink reflex. Alerting is hypothesized to nonspecifically prime motor pathways following a mildly noxious acoustic or somatic stimulus. Consistently, an intense acoustic prestimulus facilitates both reflexive and voluntary reactions. The onset of a transient stimulus in the visual periphery is hypothesized to elicit a covert movement of sensory attention to that location, a process referred to as 'reflexive orienting.' Like alerting, orienting modulates both reflexive and voluntary reactions. In the proposed research, reflexive and voluntary reactions, and their electrophysiological correlates, will be directly compared to evaluate the possibility that common mechanisms underlie modulatory effects on both classes of response. Manipulations of orienting and alerting will be employed that have proven useful in studies performed by cognitive psychologists but which have not been tried by reflex physiologists, and vice versa. In addition, experiments are planned to clarify the neural pathways mediating the brainstem reflexes that are to be used in the alerting and orienting studies, specifically, the visual blink reflex and visual prepulse inhibition. The results of these studies should be of interest to both cognitive psychologists and neuroscientists, and may find immediate application in the neurological clinic for quantitative, objective assessment of the integrity of subcortical visual pathways.